In many industrial production processes, especially production processes for chemical products, gaseous process products other than the actual product of the production process are obtained, which, before they are released as offgases into the environment, have to be processed in order to minimize economic losses and damage to the environment. Such workup steps may include various processes. Examples of such cleaning operations include scrubbing, absorption (for example of NOx), adsorption, condensation and filtration. It is customary in the prior art to combine such gaseous process products that have been subjected to preliminary cleaning in this way on conclusion of all cleaning operations conducted for preliminary cleaning (and possibly even beforehand) and to send the offgas stream thus obtained to a common offgas combustion in which the offgas is combusted sufficiently completely that the combustion gases (if required after further cleaning, for example a base scrubbing to bind acidic constituents) can be released into the environment while complying with official environmental regulations.
One example of an industrial production process in which gaseous process products are obtained is the production of isocyanates by phosgenation of the corresponding amines. In the phosgenation, a gaseous process stream comprising the hydrogen chloride co-product and unconverted phosgene is obtained. Residues of the solvent used (and also inerts or else excess carbon monoxide from the phosgene preparation) are regularly also present therein. Workup steps that follow may give further gaseous process products, some of which also contain solvent. In this connection, the recovery of solvent from the various gaseous process products obtained at different points in the process is important. Processes for solvent recovery via distillation in isocyanate production processes are addressed, for example, by EP 1 575 908 B2 and EP 1 575 906 B1. None of these publications describes solvent recovery immediately prior to the offgas combustion.
WO 2004/056758 A1 describes further purification of hydrogen chloride from hydrogen chloride/phosgene separation by adsorption of impurities (residues of phosgene and chlorobenzene) on activated carbon.
It would be desirable to have a reliable process available for solvent recovery from offgas streams from isocyanate production plants before introduction thereof into the offgas combustion.
It would also be desirable to be able to adapt such a process in a simple manner to altered boundary conditions, for example altered costs for solvent, energy or offgas combustion.